1. Field of the Invention
The present invention relates to an apparatus and a method for forming an image for printing, and more particularly to an apparatus and a method for forming image data to be printed on output paper, based on image data optically read from a document.
2. Description of the Related Art
In general, an ADF (Auto Document Feeder) in an image forming apparatus such as a copier feeds multiple documents placed on a document tray one by one into the apparatus and conveys the fed document to an image reading position. At the image reading position, an illumination unit applies light to the document. Reflected light from the document is subjected to photoelectric conversion, whereby image data is detected. The images of the documents successively fed by the conveyance are read in succession. In detecting the image data, shading correction is performed in which gray levels of pixels of the image data output from elements of the photoelectric conversion unit are made uniform.
When a CCFL (Cold Cathode Fluorescent Lamp), for example, is used as the illumination unit, it takes a long time for warm up to obtain the required quantity of emission light after the apparatus is powered on. This problem is noticeable particularly in a low-temperature environment. More specifically, the quantity of light applied from the illumination unit varies as the document reading operation continues, so that the density of the read image varies from document to document (gradually becomes lighter). This is because shading correction is usually performed only once before the start of execution of a job in a case where a plurality of documents are successively read by ADF.
Conventionally, in order to address the problem of image density variations, for example, a method is proposed by Japanese Laid-Open Patent Publication No. 11-17942. According to this method, at a time of document reading by ADF, shading correction is executed when a prescribed time has elapsed since CCD is turned on.
According to a method proposed by Japanese Laid-Open Patent Publication No. 11-355570, at a time of document reading by ADF, a timing at which shading correction is executed is changed during document reading, depending on a variety of states/settings of a scanner (the light source ON time, the quantity of emission light from a light source, the number of documents to be read).
According to a method proposed by Japanese Laid-Open Patent Publication No. 2000-278497, in a case where both sides of a document are read by ADF, shading correction is executed when the document is reversed in order to read the second side after reading of the first side is ended.
In the conventional methods as described above, shading correction is executed at a timing depending on a variety of states/settings of a scanner. In other words, shading correction is executed at such timing that ignores a manner of arranging images in a print product, which leads to the following problems.
For example, assuming that a printed product is in a booklet form, shading correction is executed while documents corresponding to a double-page spread of the booklet are read. In this case, the image of the document read before shading correction is present on the left side of the double-page spread while the image of the document read after shading correction is present on the right side of the double-page spread. As a result, a subtle difference in density/color tone arises between the images on the left side and the right side of the spread. On the other hand, with Page Layout Function in which a plurality of images read from multiple pages of documents are arranged on a surface of one sheet of output paper, a plurality of images are placed on a single output sheet of paper. Also in this case, shading correction is done between the start and the end of reading a plurality of images, so that a difference in density/color tone is caused between a plurality of images on the same sheet.
If an image density difference arises on the same sheet or between the spread pages, the difference is easily noticed and gives the users a sense of discomfort or displeasure.
It is therefore desired to provide an image forming apparatus that does not cause a density/color tone difference between the images on the left side and the right side of a double-page spread of a booklet and between a plurality of images on the same page, which are edited with printed output sheets.